US 2011/0210610 A1 discloses a photovoltaic system that comprises a plurality of parallel-connected photovoltaic generators. The parallel connection is performed in groups, in that initially individual photovoltaic generators are combined to form connection units and the currents of several of these connection units are then combined in one collection unit prior to said currents being directed to an inverter. A fault monitoring procedure is performed in the connection units for each individual photovoltaic generator but said procedure is not especially directed at detecting a ground fault or measuring the differential current.
US 2012/0048326 A1 discloses a photovoltaic system wherein a plurality of photovoltaic generators are parallel-connected in groups as in the latterly discussed prior art. In this system, the differential current is measured for each individual photovoltaic generator in a connection unit in order to monitor the photovoltaic generator to detect the occurrence of a ground fault. A further differential current measurement is performed in a common inverter for the direct current of all photovoltaic generators in order also to monitor the lines between the differential current sensors of the individual photovoltaic generators and the inverter with regard to the occurrence of a ground fault. US 2012/0049627 A1 also discloses such a monitoring with regard to the occurrence of a ground fault for of a photovoltaic system.
US 2002/0105765 A1 discloses a photovoltaic system wherein the fault current portion of a differential current in input lines of a photovoltaic inverter is determined by taking into consideration the actual capacity of the connected photovoltaic generator and by taking into consideration the potential fluctuations at the input lines with respect to ground in order to be able to monitor said fault current portion reliably with regard to the occurrence of small step changes even in the case of large variable capacitive leakage current portions in the differential current.
WO 2011/026874 A2 discloses a method for monitoring a plurality of photovoltaic generators of a photovoltaic system, which are denoted here as strings, in order to be able to recognize, for example, a partial failure of even only one substring of a photovoltaic generator at an early stage. For this purpose, currents of the individual strings are measured and simultaneously occurring currents are placed in relation to one another and the long term progression of these relationships is evaluated.
There still is a need of a method of monitoring an inverter with regard to the occurrence of a critical fault current and to provide an inverter that is suitable for performing such a method, said method and inverter rendering it possible even in the case of a high electric power output and a corresponding high capacity of connected photovoltaic generators to detect small short-term increases in the fault current just as reliably as absolute values of the entire differential current that is flowing in the inverter or of a specific current portion of this differential current.